1. Technical Field
The present invention relates to a radio device comprising a primary oscillator, a phase locked loop driven by a signal provided by the primary oscillator, and a radio emitter and/or receiver circuit receiving a radio frequency signal provided by the phase locked loop.
2. Description of the Related Art
Generally, radio emitter and/or receiver circuits need for their operation (carrier detection, modulation and demodulation) a radio frequency signal produced by an oscillator. This oscillator is, for example, of the resonance type, frequency multiplication type, or still frequency synthesis type by means of a phase locked loop.
Oscillators provided with a phase locked loop are commonly used in applications requiring several frequency channels, because the passage from a radio channel to another is then obtained by a simple change of the division range of a programmable frequency divider arranged in the phase locked loop.
Furthermore, with the evolution of silicon integration techniques for radio emitter-receiver circuits and the decrease of their cost price, there have been developed various applications of these circuits directed to the tele-transmission of data provided by measurement systems like sensors or meters, or still to the remote control of actuating systems.
As an example, FIG. 1 schematically represents a meter 1, for example a gas meter, provided with a radio device 2 for transmitting meter data in a hertzian way. The radio device 2 comprises an emitter-receiver circuit 3 receiving as an input a radio frequency signal SRF provided by a phase locked loop 4, the phase locked loop being driven by a signal Sref provided by a primary oscillator 5.
In this kind of application, the radio device 2 is generally autonomous and comprises its own electric supply source, such as a battery or a small battery set. To spare the lifetime of the supply source, the radio device 2 operates by intermittence according to a determined data transfer protocol in order to reduce the periods of radio activity. For example, the protocol consists in setting the device 2 in reception mode during a fraction of a second every 10 seconds, continuously or at some hours of the day or the month. When a data transmission request is received (tele-reading of the meter), the device switches from the reception mode to the emission mode, sends the data provided by the meter, and then comes back to the stand-by state. If, on the other hand, no call is detected (absence of carrier), the device comes back immediately to the stand-by state.
In practice, data transmission requests are infrequent and the average electric consumption of radio device 2 is essentially determined by the numerous radio listening periods. FIG. 2A represents the graph of the current which is consumed by device 2 and shows that each radio listening period PRx comprises a period Tstart for powering-on the device 2, during which device 2 switches from the stand-by state to the active state, and a period TRx, during which the device is effectively operational in the reception mode. The starting time Tstart is relatively long, in the order of some milliseconds to some tens of milliseconds. This time Tstart is in particular determined by the starting time of the primary oscillator 5, to which is added the starting and stabilisation time of the phase locked loop 4 and, once the loop 4 is locked, the time for powering-on the emission-reception circuit 3. On the other hand, the listening time TRx itself is generally very short, as the device comes back immediately to the stand-by state when the carrier RF is not detected or when the received message does not need a response.
Thus, it appears that the starting time Tstart has a non-negligible influence on the average consumption of a radio device operating in an intermittent way during short duration periods.
As illustrated in FIG. 2B, this drawback is also found in applications in which radio device 2 operates in the emission mode only and repetitively sends data supplied by meter 1, for example every 10 seconds, without knowing if these data are received by an operator. Here, the practice is to repeat the emission sequences that ensure that the data will be accessible at the moment of tele-reading. In this application as in the preceding application, each emission period PTx comprises an operational period TTx preceded by a powering-on period Tstart having a non-negligible influence on the electric consumption.
A conventional solution to reduce this drawback consists in improving the integrated circuits"" technological characteristics which have an incidence on their electric consumption and their switching speed. However, the stabilization time of a phase locked loop depends on various other parameters, in particular the characteristics of a loop filter ensuring the spectral purity of the signal SRF and is all the longer as the bandwidth of this filter is narrow. Also, a quartz oscillator presents a non-negligible starting time determined by the piezoelectric properties of quartz.
To reduce this drawback, the present invention is based on the observation that, when starting the radio device, the powering-on of some elementsxe2x80x94and the resulting current consumptionxe2x80x94is not useful, because these elements cannot become operational as long as other elements are not themselves operational. For example, in FIG. 1, the phase locked loop 4 cannot be locked as long as the primary oscillator 5 providing the reference signal Sref is not operational.
Thus, the present invention provides a radio device of the type described above comprising means for progressively powering-on the elements of the radio device according to a predetermined starting order of these elements.
According to one embodiment, the powering-on means are arranged to power-on the primary oscillator before powering-on the phase locked loop.
Preferably, when the phase locked loop comprises a phase comparator, a voltage controlled oscillator and loop elements, the powering-on means are arranged to power-on the voltage controlled oscillator before powering-on other elements of the phase locked loop.
According to one embodiment, the powering-on means comprise temporization means for determining the duration between the powering-on of the voltage controlled oscillator and the powering-on of other elements of the phase locked loop.
According to one embodiment, the powering-on means comprise means for monitoring the output of the primary oscillator and are arranged to power-on at least one element of the phase locked loop when the output of the primary oscillator delivers a signal above a predetermined threshold. According to one embodiment, the powering-on means are arranged for powering-on the radio emitter and/or receiver circuit only after the powering-on of all the elements of the phase locked loop. The powering-on means comprise for example means for detecting the locking of the phase locked loop, and are arranged to power-on the radio emitter and/or receiver circuit when the phase locked loop is locked.
The means for detecting the locking of the phase locked loop comprise for example a window comparator receiving as an input the control signal of the voltage controlled oscillator.
According to one embodiment, the phase locked loop comprises a programmable frequency divider enabling the change of the frequency of the radio frequency signal that it delivers, and the powering-on means are arranged to disable the radio emitter and/or receiver circuit when the division range of the frequency divider is changed, and power-on again the emitter and/or receiver circuit when the phase locked loop is locked upon a new frequency.
The present invention is also directed to a device, in particular a sensor or meter of a physical magnitude, an actuator, a radio-telephone, comprising a radio device as described above.